In data centers, information technology (IT) racks have been installed in which information processors such as servers are mounted. The information processors mounted in the IT racks consume electric power and generate heat corresponding to the consumed electric power. Such heat causes abnormal operation of the information processors if it is left without any treatment. In the data centers, thus, the information processors are cooled down by air conditioners.
In an example of the data centers, the information processors mounted in the IT racks are cooled down by sucking, from a space under the floor, cold air supplied by a base air conditioner. The information processors exhaust the air warmed by heat taken from the information processors. The base air conditioner sucks the warmed air exhausted from the information processors, cools down the sucked air, and supplies again the cooled air to the information processors through the space under the floor.
Recently, in the data centers, an increasing number of IT racks have been installed in which a plurality of blade servers having enhanced processing capability are mounted, for example. The amount of heat generated by the blade servers increases as the blade servers enhance the processing capability. Therefore, it may be difficult for the base air conditioner alone to sufficiently cool down the blade servers. A data center is disclosed in which task air conditioners are provided near the upper part or the side part of an IT rack including such blade servers and other equipment so that information processors generating large amounts of heat are cooled down.
Cooling of information processors by a task air conditioner is described with reference to FIG. 9. FIG. 9 is a schematic illustrating a structure of an air conditioning system according to related art. The task air conditioner sucks air exhausted from the IT rack and cools down the sucked air. The task air conditioner supplies again the cooled air to the IT rack and forms a region in which a locally circulating air flow is generated. In this way, the task air conditioner supplies cooled air, and cools down the information processors generating large amounts of heat, in addition to cooled air supplied by the base air conditioner. Examples of the related art are disclosed in Japanese National Publication of International Patent Application No. 2006-526205, Japanese National Publication of International Patent Application No. 2008-502082, and Japanese Laid-open Patent Publication No. 2006-114669.
In the related art, however, the information processors cannot be efficiently cooled down. Specifically, the task air conditioner is operated at a fixed air volume such that a temperature difference between suction air and exhaust air (ΔTlac) is from 10 to 15° C. When the thermal load of suction air is high, the thermal load may exceed the cooling capacity of the task air conditioner.
For example, the temperature difference between suction air and exhaust air of the IT rack (ΔTit) housing the information processors is designed to be from 7 to 15° C. However, ΔTit of the IT rack housing information processors designed so as to achieve low noises and low power consumption may exceed 15° C. In such a case where ΔTit is larger than ΔTlac, the thermal load sucked by the task air conditioner exceeds the cooling capacity of the task air conditioner. As a result, the task air conditioner cannot sufficiently cool down the sucked air, and causes a hot spot at which exhaust heat is locally accumulated. The hot spot results in air having a temperature higher than that of typical air being sucked in the IT rack. As a result, the information processors cannot be sufficiently cooled down.
Alternatively, the occurrence of the hot spots can be prevented by increasing the number of installed task air conditioners so as to cool down the IT track generating large amounts of heat. This method, however, lowers a load factor of the base air conditioner when the base air conditioner has a reserved cooling capacity. As a result, total air conditioning efficiency is lowered. It is difficult to say that the information processors can be efficiently cooled down.